1. Technical Field
The present invention relates to a sulfur-containing amino acid derivative. Furthermore, the present invention relates to a reagent containing the sulfur-containing amino acid derivative, an antibody recognizing an endogenous low-molecular-weight compound using the sulfur-containing amino acid derivative and a method for producing this antibody, and a method of measuring an endogenous low-molecular-weight compound using the sulfur-containing amino acid derivative.
2. Background Art
A non-competitive method for immunologically measuring low-molecular-weight compounds has been reported (JP-A-8-211054). According to this report, an animal can be immunized with an antigen-antibody complex of hapten (antigen), and an antihapten antibody (the first antibody) as an immunogen, and a new second antibody capable of recognizing the complex can be obtained. The principle of the measurement method is that the first antibody is immobilized on a carrier, an antigen-antibody reaction with hapten occurs, and then the labeled second antibody is bound thereto. However, at least two kinds of antibodies are necessary for this method, which increases the time and cost required for these types of measurements. In addition, at least two antibodies having high selectivity to the antigen need to be prepared. While there are other reports on the detection or measurement of a low-molecular-weight compound (JP-A-2000-55917, J. Immunological. Met. 272, 1-10 (2003), Histochemistry 90, 427-445 (1989) and Olin. Chem. Lab. Med. 42, 1377-1383 (2004)), multiple kinds of antibodies are necessary, as in the above-mentioned methods, and sufficient sensitivity to permit quantification of a low-molecular-weight compound cannot be obtained.
There are two factors that can prevent production of an antibody with sufficient affinity for a low-molecular-weight compound, particularly an endogenous low-molecular-weight compound. One of them is the size of epitope. Generally, the epitope size is thought to be about 3-8 amino acid residues. Therefore, when the low-molecular-weight compound is an amino acid, it is difficult to obtain an anti-amino acid antibody recognizing, for example, one amino acid residue, and even if it can be obtained, the affinity is considered to be low. The other factor is that an immune response to an endogenous compound such as amino acid does not generally occur easily.
Generally, there are two types of immunoassays that can be used to examine an antigen-antibody reaction: a competitive immunoassay and a non-competitive immunoassay. In the competitive immunoassay, an antigen in a sample and a given amount of a labeled antigen competitively bind to an antibody immobilized in a given amount on a carrier. That is, when the amount of an antigen in a sample is small, the amount of binding between a labeled antigen and the antibody increases, and high signal intensity is observed. When the amount of the antigen in a sample is high, low signal intensity is observed conversely. On the other hand, as a non-competitive immunoassay, the sandwich immunoassay is known. In the sandwich immunoassay, an antigen-containing sample is added to excess amount of an antibody (the first antibody) immobilized on a carrier and then a labeled antibody (the second antibody) is added. While the first and second antibodies recognize the same antigen, since each antibody recognizes a different site of the antigen surface, one antigen is sandwiched between the two antibodies to form a sandwich-type binding pattern. As a result, the signal increases consistent with the amount of the antigen, which enables a quantitative analysis of the antigen amount.
In general, in a competitive immunoassay, a reaction time of a few hours to one night is required for bonding form (B) and free form (F) to reach equilibrium in an antigen-antibody reaction. In a competitive immunoassay, when an unlabeled antigen and a labeled antigen show different affinities for an antibody, the accuracy of the analysis values may decrease.
In contrast, in a non-competitive immunoassay, the reaction time can be shortened, and a measurement method with a wide measurement concentration range can be constructed since the antigen amount corresponds to the amount of the labeled antibody. During an antigen-antibody reaction, two antibodies are present in excess amounts relative to the antigen amount. Thus, the equilibrium shown by the formula (1) is directed toward formation of a complex, under which environment even a trace amount of antigen can form a complex with ease. As a result, a non-competitive immunoassay enables highly sensitive measurements as compared to a competitive immunoassay.[antigen][antibody][antigen-antibody]  (1)
However, as mentioned above, in a sandwich type non-competitive immunoassay, two kinds of antibodies are necessary for one antigen. These antibodies need to recognize different regions of the antigen. Therefore, when an antigen is a low-molecular-weight compound, it is difficult to obtain two antibodies with different recognition sites due to the small molecular size. Thus, it is difficult to use a non-competitive immunoassay for the measurement of a low-molecular-weight compound.